As drilling operations are conducted in deeper and deeper water, such as 600 feet and more, it becomes impractical and uneconomical to drill from platforms supported by the underwater ground. These drilling operations are better conducted by floating ship shape vessels and semi-submersibles. The mooring system or station keeping system for ship shape drilling vessels is more critical than for semi-submersibles. A semi-submersible is basically symmetrical; therefore, the forces and motions imparted by wind, wave and current are generally the same regardless of direction; however, the forces, lateral displacements and motions imparted by beam seas are many times greater than those imparted by head seas for ship shape hulls. It is for these reasons that the anchor system for a ship shaped hull must not only correct for lateral displacement as required for semi-submersibles but also have the capability of warping the vessel so as to head it into the sea thus avoiding large lateral displacements, motions and forces encountered in beam seas. While ship shape vessels can move from one location to another at normal ship speeds, which is substantially faster than semi-submersibles, their ship speeds advantage is lost unless there is maximum onstream time of the ship shape hull by the higher theoretical utilization factor of a semi-submersible. It can readily be seen that in order to facilitate operations, increase efficiency and on-stream time, the mooring system should permit maximum flexibility in turning the vessel into the sea and maintaining the vessel within permissible alignment limits over the well.
In addition, there are a number of operational problems in prior mooring systems. For example, brake bands on the windlass wildcats are not cooled and burn up easily in controlling the payout of the anchor chains, since no means are provided, except the brake bands, for this purpose. The chain stoppers cannot be released while under a full load in emergency without severe damage to them. There is inadequate sensing of forces, static and dynamic, which are imparted to the vessel and absorbed by the mooring system including tension read-outs which are critical to the operation of an anchor station keeping system.
The following U.S. patents illustrate various methods and apparatus for mooring floating vessels, anchoring systems, sensing means and other components of mooring systems: Nos. 2,881,591; 2,986,889; 2,987,892; 3,031,997; 3,191,201; 3,279,404; 3,402,687; 3,422,783; 3,536,024; 3,552,343; 3,580,207; 3,601,075; 3,602,175; 3,605,668; 3,613,625; 3,620,181; 3,670,813; 3,702,105; 3,774,562; 3,805,728; and 3,822,663. None of these patents, however, discloses the mooring system of the present invention as well as its improved components and various features by which efficiency and onstream time is increased, maximum flexibility is provided in turning the vessel into the sea and the abovementioned operational problems are minimized.